The question has been posed whether plastic sheeting is used as a vapor diffusion retarder (VDR). ABSOLUTELY! There are different types of vapor diffusion retarders- but plastic sheeting is one of the primary ways in which to control the moisture content coming into your home or building. A vapor diffusion retarder is a material that reduces the rate in which water vapor can move through a material such as a plastic sheeting liner or vapor barrier. The ability of a material to retard the diffusion of water vapor is measured in units referred to as "perms. Vapor barriers are also measured in terms of its "permeability". Any material with a perm rating of less than 1.0 is considered a vapor diffusion retarder, or a plastic sheeting vapor barrier.
Some plastic sheeting is more effective as a vapor diffusion retarder. That is because it is engineered to specifically retard the flow of water. Additionally these plastic sheeting liners are tested to the ASTM E1745 standard.